IDAHO FALLS, Idaho (AP) -- Officials with the U.S. Department of Agriculture say a microscopic parasite that damages potato crops has been found in three more eastern Idaho fields, bringing the number of infected fields to 15.

Tina Gresham with the USDA in Idaho Falls, said though pale cyst nematode infestations continue to be discovered in eastern Idaho, efforts to fight the pest appear to be helping.

"There's a possibility that there could be more (infested) fields," she told the Post Register (http://bit.ly/KaRXdF). "At the same time, I do believe there is a light at the end of the tunnel."

The nematode is a microscopic parasite that attacks the roots of potato plants. It doesn't harm the potato itself and poses no threat to humans or animals, but it can devastate crop yields.

It was first found in eastern Idaho in 2006, and since then 15 infected fields have been discovered in Bonneville and Bingham counties.

Recent tests in five of those fields, however, uncovered only dead nematodes, Gresham said. Researchers will continue to test the soil in the fields for two years to make sure the pests are wiped out.

USDA spokesman Larry Hawkins said that since 2007, the agency has spent about $7 million a year to try to get rid of the nematodes in eastern Idaho fields.

Gresham believes this year's newly discovered infestations began before the USDA eradication program was started. Besides fumigating the fields themselves, the agency is urging farmers to clean equipment before moving it between fields.